Conditional Moment Generating Function of a Negative Binomial












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Suppose X conditioned on K follows a Negative Binomial Distribution i.e. $X|K = k_i sim NB(r-k_i, q)$, where $r$ is a constant and $K sim Bin(m, q)$. I'm trying to calculate the MGF of X. So far, I have done the following:



$$M_x(s) = E[s^{sX}] = E[E[e^{sX}|K]] = E[(frac {q}{1-(1-q)e^s} )^{r-K}]$$



Since K itself is Binomial, the algebra in calculating the above expected value is complex. Is there any identity or any algebra trick that I can use to get a decent expression for MGF of X?










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    0












    $begingroup$


    Suppose X conditioned on K follows a Negative Binomial Distribution i.e. $X|K = k_i sim NB(r-k_i, q)$, where $r$ is a constant and $K sim Bin(m, q)$. I'm trying to calculate the MGF of X. So far, I have done the following:



    $$M_x(s) = E[s^{sX}] = E[E[e^{sX}|K]] = E[(frac {q}{1-(1-q)e^s} )^{r-K}]$$



    Since K itself is Binomial, the algebra in calculating the above expected value is complex. Is there any identity or any algebra trick that I can use to get a decent expression for MGF of X?










    share|cite|improve this question









    $endgroup$















      0












      0








      0





      $begingroup$


      Suppose X conditioned on K follows a Negative Binomial Distribution i.e. $X|K = k_i sim NB(r-k_i, q)$, where $r$ is a constant and $K sim Bin(m, q)$. I'm trying to calculate the MGF of X. So far, I have done the following:



      $$M_x(s) = E[s^{sX}] = E[E[e^{sX}|K]] = E[(frac {q}{1-(1-q)e^s} )^{r-K}]$$



      Since K itself is Binomial, the algebra in calculating the above expected value is complex. Is there any identity or any algebra trick that I can use to get a decent expression for MGF of X?










      share|cite|improve this question









      $endgroup$




      Suppose X conditioned on K follows a Negative Binomial Distribution i.e. $X|K = k_i sim NB(r-k_i, q)$, where $r$ is a constant and $K sim Bin(m, q)$. I'm trying to calculate the MGF of X. So far, I have done the following:



      $$M_x(s) = E[s^{sX}] = E[E[e^{sX}|K]] = E[(frac {q}{1-(1-q)e^s} )^{r-K}]$$



      Since K itself is Binomial, the algebra in calculating the above expected value is complex. Is there any identity or any algebra trick that I can use to get a decent expression for MGF of X?







      conditional-expectation binomial-distribution negative-binomial






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      asked Dec 5 '18 at 2:44









      Resting PlatypusResting Platypus

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